1. Field of the Invention
The present invention relates to a surface acoustic wave device for use in, e.g., a resonator and a band filter, and more particularly, to a surface acoustic wave device in which a piezoelectric substrate includes a LiNbO3 film.
2. Description of the Related Art
Recently, a surface acoustic wave device adapted for higher frequencies has been demanded with a trend toward communication equipment operating in a higher frequency range. Also, a wider frequency band has been demanded in a surface acoustic wave filter.
The acoustic velocity of the surface acoustic wave and the electromechanical coupling coefficient k2 are required to be higher or larger in order to achieve a higher frequency and a wider frequency band during operation.
Japanese Unexamined Patent Application Publication No. 10-322158 discloses a surface acoustic wave device using a piezoelectric substrate in which a (100) LiNbO3 thin film is formed on a (012) sapphire substrate. Japanese Unexamined Patent Application Publication No. 10-322158 states that the electromechanical coupling coefficient can be increased by forming the (100) LiNbO3 thin film on the (012) sapphire substrate.
Japanese Unexamined Patent Application Publication No. 9-219632 discloses a surface acoustic wave device illustrated in FIG. 62. In this surface acoustic wave device 1001, a LiNbO3 single-crystal layer 1003 is formed, instead of being a thin film, on a diamond substrate 1002. An IDT electrode 1004 is formed on the LiNbO3 single-crystal layer 1003. In such a structure, given that the thickness of the LiNbO3 single-crystal layer 1003 is t1 (μm) and the wavelength of a surface acoustic wave in the n-th order mode is λn (μm), kh1=2π(t1/λn) and the Euler angles of the LiNbO3 single-crystal layer 1003 are set to fall within respective specific ranges. Japanese Unexamined Patent Application Publication No. 9-219632 states that, when those conditions are satisfied, the propagation velocity of the surface acoustic wave can be increased and the electromechanical coupling coefficient k2 can also be increased.
In the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 10-322158, the piezoelectric substrate including a (100) LiNbO3 thin film stacked on the (012) sapphire substrate is used, but the range of Euler angles of the LiNbO3 thin film at which the electromechanical coupling coefficient can be increased is limited to a narrow range.
On the other hand, the surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 9-219632 has the structure in which the LiNbO3 single-crystal layer 1003 is stacked, instead of being a thin film, on the diamond substrate 1002, so as to increase the acoustic velocity of the surface acoustic wave and the electromechanical coupling coefficient. However, the diamond substrate 1002 made of single-crystal diamond is expensive and cost reduction is demanded.